1. Remote Radio Head (RRH) Technology, Centralized Base Transceiver Station (CBTS) and Signal Transmission
As illustrated in FIG. 1A, in mobile communication systems, wireless access network is typically composed of Base Transceiver Stations (BTSs) and Base Station Controllers (BSCs) or Radio Network Controllers (RNCs) for controlling a plurality of BTSs. Wherein the BTS is mainly composed of a baseband processing subsystem, a radio frequency (RF) subsystem, and an antenna etc and is responsible for transmitting, receiving, and processing wireless signal. A BTS can cover various cells by means of a plurality of antennas, as illustrated in FIG. 1B.
In mobile communication systems, there are wireless network coverage problems that are more difficult to solve with conventional BTS technologies, such as, indoor coverage of high-rise buildings, coverage hole, or the coverage of shadow zone, RRH technology is a more effective solution being proposed to solve the above problems. In the BTS system using RRH technology, the primary radio frequency units and antennas are installed in regions that are required to provide coverage, and are connected to other units in the BTS through wideband transmission lines.
This technology can be further developed to a CBTS technology that uses RRH technology. Compared with the conventional BTS, the CBTS using RRH technology has many advantages: the centralized structure allows to use several Micro-Cells to replace a Macro-Cell based on the conventional BTS, therefore it can be adapted to various wireless environment better, and enhance wireless performances such as system capacity and coverage etc; the centralized structure enables the replacement of soft handoff in the conventional BTS by softer handoff, therefore obtains additional processing gain; the centralized structure also enables valuable baseband signal processing resources to become a resource pool shared by several cells, therefore has the advantage of Statistic Multiplex, and also decreases system cost. The following patents disclose some implementation details about the CBTS using RRH technology, they are: U.S. Pat. No. 5,657,374, filed on Mar. 23, 1995, entitled “Cellular system with centralized base stations and distributed antenna units”, and U.S. Pat. No. 6,324,391, filed on Jun. 28, 1999, “Cellular communication with centralized control and signal processing”, which are hereby incorporated by reference.
As illustrated in FIG. 2, the CBTS system 200 using RRH technology is composed of a central channel processing subsystem 201 and a plurality of Remote Radio Units (RRUs) 2041, 2042, . . . , 204M installed integrately. They are connected to each other through wideband transmission links or network. The central channel processing subsystem 201 is mainly composed of functional units such as a channel processing resource pool 202 and a signal route distribution unit 203, etc. The channel processing resource pool 202 is formed by stacking a plurality of channel processing units 2021, 2022 . . . , 202N together and is used to perform base band signal processing, etc. The signal route distribution unit 203 dynamically distributes the channel processing resources in accordance with different cell traffics to achieve efficient share of a plurality of cell processing resources. The signal route distribution unit 203 can be disposed outside of the CBTS as a separate equipment other than be disposed inside the CBTS as illustrated in FIG. 2. The RRUs 2041, 2042, . . . 204M are mainly composed of functional units such as radio frequency power amplifiers in transmission channel, low noise amplifiers in receiving channel, and antennas, etc (not shown entirely). Typically, the links between the central channel processing subsystem 201 and the Remote Radio Units (RRUs) 2041, 2042 . . . , 204M can use such transmission media as optical fiber, copper cable, microwave, etc.
In the two BTS systems using RRH technology discussed above, the key problem to be solved is the wireless signal transmission between the RRU(s) and the host BTS. The host BTS herein represents units included in the BTS and including baseband processing unit except the radio frequency unit in the two BTS systems using RRH technology discussed above. Typically, analog intermediate frequency or analog radio frequency signal transmission scheme is adopted. Although it is easier to adopt analog signal transmission scheme, there will be disturbing components, for example noise, etc, in analog lines, and the modulation of the transmitted signal will introduce nonlinear distortion, in addition, the analog transmission may decrease the utilization of transmission line, and hamper the implementation of large capacity multiplex technology, therefore, it is difficult to adopt the analog transmission scheme in large scale networking.
To solve the problems, the scheme of digital signal transmission is proposed in the following two patents: Chinese patent application CN1464666, entitled “A soft BTS system based on remote fiber and its synchronization method” filed on Jun. 11, 2002, and Chinese patent application CN1471331, entitled “The BTS system in mobile communication” filed on Jul. 2, 2003 (the priority date being Jul. 2, 2002), the scheme of digital base band signal transmission is generally used to decrease the requirement for transmission bandwidth as much as possible. However, CN1464666 disclosed only the simple method of using optical fiber to transmit digital I/Q (In-phase/Quadrature) baseband signal between the RRU and the host BTS, that is, the digital I/Q base band signal is converted to serial data stream by means of parallel to serial conversion at the transmitting end, and then transmitted to the receiving end via an optical transmitter, and restored to the digital I/Q base band signal by means of serial to parallel conversion after received by an optical receiver and the receiving end. CN1471331 proposed a transmission technology of using Ethernet technology in physical layer, the technology uses continuous bit stream format specially defined instead of Ethernet MAC (Media Access Control) frame. At present, a corporation organization named CPRI (Common public Radio Interface) is also engaged in the standardization of the digital baseband transmission between the RRU(s) and the host BTS, and its technology specification can be obtained from its website. The technology specification adopts a technology similar to that adopted in CN1471331, that is, physical interface uses 1000 MB or 10 GB Ethernet standard, upper layer uses a continuous bit stream format user-defined, but CPRI only supports star networking in the form of point to point, whereas CN1471331 can support the link converge based on hub.
On the other hand, SDH (Synchronous Digital Hierarchy) and OTN (Optical Transmission Network) based on Wavelength Division Multiplex technologies such as DWDM (Dense Wavelength Division Multiplex)/CWDM (Coarse Wavelength Division Multiplexing) have been widely used in backbone network and wideband Metropolitan Area Network (MAN), but the existing technology of digital transmission between the RRU(s) and the host BTS uses special transport protocols and specification, and therefore, it is difficult to use the existing maturate wideband transmission resource in the existing telecommunication network, so the networking cost is increased, moreover, there are problems, such as nonflexible networking and complicated maintenance and management, in the existing technology of digital transmission between the RRU and the host BTS.
2. Generic Framing Procedure (GFP)
Generic Framing Procedure (GFP) is jointly recommended by ITU-T and ANSI, it is used to adapt the data stream of block coding or packet types to continuous byte synchronization transmission channel, typically for example the new technologies as SDH (Synchronous Digital Hierarchy) and OTN (Optical Transmission Network), the detailed technology specification of which may be referred to ITU-T G.7041 or ANSI TIX1.5/2000-024R3, which are hereby incorporated by reference. GFP can be classified into a frame mapping GFP (GFP-F) that supports PDU (Protocol Data Unit) and a transparent GFP (GFP-T) that supports block coding. The GFP-F can be used in the adaptation of protocol packet as PPP (Point to Point Protocol), MPLS (Multi-Protocol Label Switching), and Ethernet MAC (Media Access Control), etc, and the GFP-T can be used to directly adapt block coding character stream in 1000 MB Ethernet line, etc., accordingly, some application requirements for very little time delay can be satisfied, but the utilization of the GFP-T transfer bandwidth is lower than that of GFP-F transfer bandwidth.
In FIG. 3, a frame structure of GFP-T type is illustrated schematically. The GFP-T frame is composed of a core header and a payload part, and the payload part includes a payload header, payload and a selectable payload FCS (Frame Check Sequence). The core header includes a PL1 field indicating the payload length and a core header error control field (cHEC) which is functioned as GFP frame delimitation similar to ATM (Asynchronous Transfer Mode) Cell delimitation as well as provides error protection for the core header. The payload header indicates payload types and provides error protection by the cHEC. The Payload Type Identifier (PTI) indicates that the GFP-T frame carries user data when it is “000”, and indicates that the GFP-T frame carries client management information when it is “100”, while the payload FCS indicator (PFI) indicates if there is a payload FCS. User Payload Identifier (UPI) and the PTI together indicate the types of user data or client management information in the payload, as illustrated in table 1 and 2.
TABLE 1PTI = 000UPI valueGFP frame payload0000, 0000, 1111, 1111Reserved unused0000, 0001Frame mapping Ethernet MAC0000, 0010Frame mapping PPP0000, 0011Transparent optical fiber channel0000, 0100Transparent FICON0000, 0101Transparent ESCON0000, 0110Transparent GB Ethernet0000, 0111reserved0000, 1000Frame mapping MAPOS0000, 1001~1110, 1111Reserved for future standard1111, 000~1111, 1110Reserved for exclusive use
TABLE 2PTI = 100UPI valueGFP frame payload0000, 0000, 1111, 1111Reserved unused0000, 0001Client signal failure (lose client signal)0000, 0010Client signal failure (lose client charactersynchronization)0000, 0011, 1111, 1110Reserved for future use
In addition, Extension Header Identifier (EXI) indicates the presence of a selectable extension header and its type, at present, a typical use of the extension header is providing channel identifier (CID), therefore supporting the multiplex of multiple separate client signals. As illustrated in FIG. 3, the payload in the GFP-T frame is super block with fixed length which is formed sequentially by 64B/65B code block. Since the direct adaptation of the transparent GFP now uses block coding character stream of a 8B/10B line code, 64B/65B code block includes user data character and control character, so a flag bit is used to indicate if there is a control character in the 64B/65B code block (the bit indicated by L1, L2, . . . , L8 in FIG. 3), wherein the high 4 bits of the control character are used as the following control character indication and the position indication of the control code in the original 8B/10B code stream, and the low 4 bits are used to transmit the control code itself.
To ensure the transmission of the client signal, the bandwidth of the transmission channel, for example SDH/OTN, etc, should be a little wider than the bandwidth required by the GFP frame, since the size of each super block in the GFP-T frame is 536 bits, the bit length of the GFP-T frame, L can be denoted as:L=Loverhead+536·N  (1)wherein N is the number of the super blocks in the GFP-T frame, Loverhead is the overhead length of the core header, the payload header and the selectable payload FCS, etc, in the GFP-T frame. If the original client signal rate is BC bit/s, and the bandwidth of transmission channel for example SDH/OTN, etc, is BT bit/s, considered that each super block can carry a client signal stream of 512 bits, the number of the super blocks in the GFP-T, N should satisfy the following expression:
                              L                      B            T                          <                              512            ·            N                                B            C                                              (        2        )            
So the minimum number of the super blocks required by the GFP-T frame to satisfy the above condition, N is:
                              N          min                =                  ⌈                                                    B                C                            ⁢                              L                overhead                                                                    512                ⁢                                                                  ⁢                                  B                  T                                            -                              536                ⁢                                                                  ⁢                                  B                  C                                                              ⌉                                    (        3        )            
Wherein symbol ┌x┐ indicates the minimum integer larger than or equal to x.
3. Virtual Concatenation (VCAT) Technology
The STM-N/OTM-n standard transmission link of SDH/OTN is formed by multiplexing some typical Virtual Containers (VCs) with fixed rate according to certain multiplex rules. For example, the basic VCs of SDH include VC-11, VC-12, VC-2, VC-3, and VC-4, while VC-4 can further form four VCs with higher rate: VC-4-4c, VC-4-16c, VC-4-64c, and VC-4-256c by means of sequential concatenation, as illustrated in table 3.
TABLE 3VC typeVC bandwidthVC payload bandwidthVC-111664Kbit/s1600Kbit/sVC-122240Kbit/s2176Kbit/sVC-26848Kbit/s6784Kbit/sVC-348.960Mbit/s48.384Mbit/sVC-4150.336Mbit/s149.760Mbit/sVC-4-4c601.344Mbit/s599.040Mbit/sVC-4-16c2405.376Mbit/s2396.160Mbit/sVC-4-64c9621.504Mbit/s9584.640Mbit/sVC-4-256c38486.016Mbit/s38338.560Mbit/s
The technology of using finite number of fixed rate VCs has simplified SDH multiplex design, and made it easier to realize Add/Drop, multiplex and digital cross connect, but since a plenty of paddings are needed to adapt specific VC rate, the transmission efficiency is influenced. Whereas the Virtual Concatenation (VCAT) technology allows for providing more selections on transmission bandwidth by inversely multiplexing a plurality of VCs having the same rate, so the problems with transmission efficiency are solved. But since respective VC arrives at the receiving end via separate transmission paths, certain buffer is needed at the receiving end to eliminate the difference due to transmission delay.